Surgeons at Moorfields Eye Hospital in London will treat 12 patients with Stargardt’s macular dystrophy, a major cause of blindness in young people. Patients with Stargardt’s gradually lose retinal pigment epithelial cells, or RPE cells, which are essential for maintaining the proper environment that encourages retinal cells to grow.

Patients in the U.K. trial will receive RPE cells grown from human embryonic stem cells. This is the second human trial to use such cells; the first was approved by the U.S. Food and Drug Administration (FDA) last November, which we described here. In both studies, the cells will be made by Massachusetts-based Advanced Cell Technology (ACT).

The purpose of the trial is to establish the safety of the treatment, so the dozen enrolled patients are at the most advanced stages of the incurable disease. They will receive anywhere from 50,000 to 200,000 cells, injected directly into the eye, behind the retina. Doctors expect that the transplanted cells will then start to grow in their new environment, replacing the diseased and dying ones that have robbed the patients of their vision.

The eye is a good site for testing such a treatment, since it is relatively protected from the immune system; the new cells may therefore be protected from destruction by overzealous defensive cells. The isolated nature of the eye’s vitreous cavity, where the cells are injected, also makes it relatively easy for doctors to destroy the transplanted RPE cells should something go wrong, such as if the cells should start forming tumors or migrate to other areas.

The eye trials follow the first ever approved human trial of an embryonic stem cell based-therapy, approved in January 2009 in the U.S. to treat spinal cord injury. The FDA put that trial on hold after some animals receiving the stem cells developed cysts, but the company conducting the trial, Geron, provided additional safety data and new tests for detecting the unusual growths. Last summer the trial was once again allowed to move forward.

Embryonic stem cells could become an important source of replacement cells for a number of different diseases, including diabetes and Parkinson’s. As Dr. James Barinbridge, a retinal surgeon at Moorfields and the UCL Institute of Ophthalmology, told the U.K.’s Guardian, “The ability to regenerate retinal cells from stem cells in the laboratory has been a significant advance and the opportunity to help translate such technology into new treatments for patients is hugely exciting. Testing the safety of retinal cell transplantation in this clinical trial will be an important step towards achieving this aim.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.